1. Field of the Invention
The present invention generally relates to an optical probe and an optical tomography apparatus. More specifically, the invention is directed to a probe having an optical scanning function in a circumferential direction with respect to the longitudinal axis of the optical probe, and an optical tomography apparatus for obtaining an optical tomography image of a measuring object by OCT (Optical Coherence Tomography) measurement using the optical probe.
2. Description of the Related Art
As one of the methods for obtaining tomography images of measuring objects, such as living tissues and the like, a method for obtaining a tomography image by OCT measurement is proposed. The OCT measurement is one of the types of optical interference measurement, and a low coherence light outputted from the light source is split into measuring and reference light, and the measuring light is irradiated onto a measuring object, then the reflected light from the measuring object or backscattered light when the measuring light is irradiated thereon is combined with the reference light, and an optical tomography image is obtained based on the intensity of the interference light of the reflected light and the reference light. Hereinafter, reflected light from a measuring object and backscattered light are collectively referred to as the “reflected light”.
The OCT measurement is largely grouped into TD-OCT (Time Domain OCT) measurement and FD (Fourier Domain)-OCT measurement. The TD-OCT measurement is a method for obtaining a reflected light intensity distribution corresponding to a position in the depth direction (depth position) of a measuring object by measuring interference light intensity while changing the optical path length of the reference light.
In contrast, the FD-OCT measurement is a method for obtaining a reflected light intensity distribution corresponding to a depth position of a measuring object by measuring interference light intensity with respect to each spectral component of the light without changing the optical path length of the reference light, and performing frequency analysis, typically a Fourier transform, on the obtained spectral interference intensity signals using a computer. The FD-OCT does not require the mechanical scanning used in TD-OCT, so that it has been drawing wide attention as a method that allows high speed measurement.
Typical systems that use FD-OCT measurement are SD-OCT (Spectral Domain OCT) and SS-OCT (Swept Source OCT) systems. The SD-OCT system uses broadband and low coherence light and forms an optical tomography image by splitting interference light into respective frequency components using a spectroscopic device, measuring the intensity of the interference light with respect to each frequency component using an array detector, and performing Fourier transform on the obtained spectral interference signals using a computer.
Generally, each type of optical tomography apparatus described above obtains a tomography image along a certain face of a measuring object. This requires that the measuring light is scanned at least one-dimensional direction on the measuring object. As one of the means for performing such scanning, a dental probe having a measuring window on the side and performing scanning in the depth and horizontal directions by outputting a light beam from the measuring window is known as described, for example, in Japanese Unexamined Patent Publication No. 2004-347380.
In addition, it has been studied to combine each type of the optical tomography apparatus described above with an endoscope for in vivo application, and an optical probe for OCT measurement which is insertable into the forceps channel of an endoscope like that as described in Japanese Unexamined Patent Publication No. 11 (1999)-056786 is known. The optical probe described in Japanese Unexamined Patent Publication No. 11 (1999)-056786 includes an elongated tubular sheath serving as the outer tube of the optical probe, a coil shaft rotatably provided around a longitudinal axis inside the sheath, a rotary drive device for providing rotational power to the coil shaft, and an optical fiber provided inside the coil shaft. The probe is designed to perform optical scanning in a circumferential direction of the sheath by outputting light from the circumferential surface of the sheath.
The combination of the optical tomography apparatus and endoscope described above is useful for obtaining an optical tomography image of a living tissue, and hence development of optical probes for OCT measurement to be inserted into the forceps channels of endoscopes is in progress.
In OCT measurement, a tomography image is obtained based on the signals generated by the reflection light from a measuring object, high intensity reflection light results in high S/N ratio, thereby high quality tomography image may be obtained. If air is present between an optical probe outputting measuring light and a measuring object, however, the reflection light from the measuring object is reduced due to reflection at the interface between the optical probe and the air and interface between the air and the measuring object caused by the difference in the refractive indices thereof. For a measuring object having a water-soluble membrane, such as gastric wall or the like, most of the measuring light is scattered on the surface of the membrane, so that only a small amount of the measuring light reaches the inside of the measuring object, resulting in a degraded S/N ratio. Further, if a liquid, such as water or bodily fluid, is present between the optical probe and the measuring object, the measurement is affected by the light absorption or dispersion by the liquid causing an error. In order to eliminate the problems described above, it is desirable that the optical probe is brought into close contact with the measuring object so that the air or liquid is removed from the gap between the optical probed and measuring object.
In the mean time, the outer tubes of optical probes to be inserted into the forceps channels of endoscopes are commonly formed in a cylindrical shape as described in Japanese Unexamined Patent Publication No. 11 (1999)-056786 due to manufacturing reasons and compatibility with the shapes of the forceps channels. For the cylindrical optical probe with its cross-sectional outer shape being circular, the contact area with a measuring object becomes very small, which gives rise to a problem that a high quality tomography image is obtained only within a narrow area.
It may be conceivable to apply the probe having a measuring window described in Japanese Unexamined Patent Publication No. 2004-347380 to an endoscope probe. In this case, however, the direction of the only one measuring window formed on the side needs to be adjusted so as to appropriately face the measuring object. This will give rise to problems that it requires complicated manipulation, and rapid measurement is prevented.